1. Field of the Invention
The present invention relates to an electron gun for a color cathode ray tube (CRT), and more particularly, to a dynamic focus electron gun for a color CRT having improved electron beam apertures, arranged in an in-line arrangement and forming a quadrupole lens.
2. Description of the Related Art
In general, an electron gun for a color cathode ray tube (CRT), installed on a neck portion of the CRT, emits thermal electrons. The performance of a CRT is influenced by a state in which electron beams land on a phosphor screen. Thus, various types of electron guns which can improve focusing characteristics so that electron beams emitted from the electron gun accurately land on a focus of a phosphor screen, and can reduce aberration of an electronic lens, have been developed. In particular, in order to reduce the electric length of a CRT, recently, the deflection angle of the CRT has been made larger and the length of the electron gun has decreased. In this case, the focused length of an electron beam landing on a central portion of the phosphor screen is relatively longer than that of an electron beam landing on a peripheral portion of the phosphor screen, so that the focusing characteristics of electron beams in the periphery of the screen become deteriorated.
Also, since the distortion of an electron beam increases exponentially as the angle of incidence of the electron beam with respect to the phosphor screen is reduced, the beam spot diameter of the electron beam landing on the phosphor screen becomes larger. An in-line color CRT based on self-convergence includes a deflection yoke for forming a non-homogenous magnetic field for deflecting electron beams emitted from an electron gun. The electron beams emitted from the electron gun are converged throughout the whole screen by beam concentration by a main lens installed in the electron gun and a nonuniform magnetic field produced by a horizontal deflection magnetic field of a pincushion shape and a vertical deflection magnetic field of a barrel shape.
As described above, the electron beams traveling the nonuniform magnetic field are subject to both an astigmatism and a deflection force in a direction in which the beams are vertically over-focused, as shown in FIG. 8.
An example of an electron gun for a color CRT for solving the above-mentioned problem is disclosed in U.S. Pat. No. 4,814,670.
The disclosed electron gun, as shown in FIG. 1, has three electron beam through-holes 12 which are oblong in a vertical direction and located on an entrance face of a first focusing grid 11, forming a quadrupole lens. Also, a horizontally elongated electron beam through-hole 14 which three electron beams commonly pass through and located on an exit face of a second focusing grid 13, opposing the first focusing grid 11, forms the quadrupole lens. A focusing voltage VF, which is a static voltage, is applied to the first focusing grid 11 and a dynamic voltage VD, which is synchronized with a deflection signal, is applied to the second focusing grid 13.
Since the aforementioned conventional electron gun has a single horizontally elongated electron beam through-hole 14 in the second focusing electrode 13, the intensities of the electronic lens formed by the first and second focusing grids 11 and 13, that is, magnifications, are different from each other at the central portion and either side of the electronic lens. Thus, the spot sizes of electron beams landing on left and right sides of the screen become different. In particular, since one single electron beam through-hole 14, which is elongated in a horizontal direction, is located at the second focusing grid 13, assembly of an electron gun using a zig is quite difficult. Also, since vertical focusing power becomes relatively weak due to the horizontally elongated electron beam through-hole 14 at the entrance face of the second focusing grid 13 in the course of forming the quadrupole lens, a higher dynamic voltage should be applied to the grids for attaining a predetermined vertical focusing power.
Another example of a conventional electron gun is disclosed in U.S. Pat. No. 5,027,043.
The disclosed electron gun includes means for diverting an electron beam from a straight line path. The beam diverting means is used as part of a quadrupole lens for correcting astigmatism introduced by an associated self-converging yoke. The quadrupole lens is constructed such that different voltages are applied to the electrodes having vertically elongated electron beam apertures or horizontally elongated electron beam apertures.
The above-described electron gun can statically converge three electron beams arranged inline and can correct the cross section due to vertical and horizontal deflection magnetic fields for deflecting electron beams. A difference in the focal distance between the periphery and center of a screen is increased and the astigmatism due to a deflection yoke increases. The CRT can have a wide angle of deflection and be flattened, but the electron gun requires a stronger power for correcting the astigmatism and focal distance distance.
In order to attain a stronger power for correcting astigmatism, there must be a large difference in the potential applied between electrodes which form a quadrupole lens. Also, since a stronger power for correcting focal distance in the screen periphery is necessary, higher voltages must be generated at the screen periphery. However, the necessity of higher voltages causes problems of circuit reliability and voltage resistance. Also, in the case where an electron beam is incident into the screen periphery, a horizontal halo may undesirably increase at the screen periphery due to a horizontally decreased, vertically increased angle of incidence of the electron beam by the horizontally convergent, vertically divergent action of the quadrupole lens close to a main lens.
In order to compensate for distortion of a beam at the periphery of the CRT having a wide angle of deflection at a low voltage, it is necessary to constitute a quadrupole lens sensitive to voltage. To this end, the electron beam holes, which form the quadrupole lens, are more effectively made smaller. However, if the electron beam holes are smaller than those of other assembled electrodes, in view of the characteristic of an electron gun assembling process using electron beam holes, the assembling process becomes difficult, and the precision and manufacturing process are undesirably complicated.
To solve the above problems, it is an object of the present invention to provide an electron gun for a color cathode ray tube, which can prevent spots of electron beams landing on the periphery of a phosphor screen by making the angle of deflection wider, can compensate for distortion of the cross section of an electron beam due to deflection magnetic field of a deflection yoke, and can improve assembling efficiency.
It is another object of the present invention to provide an electron gun for a color cathode ray tube, which can improve the resolution at the periphery of a phosphor screen by improving focusing characteristics of electron beams and can improve voltage resistance and reliability.
To accomplish the first object of the present invention, there is provided an electron gun for a color cathode ray tube including cathodes, a control electrode and a screen electrode, forming a triode, first, second, third and fourth focusing electrodes, sequentially installed from the screen electrode and forming at least one first quadrupole lens, fifth and sixth focusing electrodes, installed adjacent to the fourth focusing electrode and forming at least one second quadrupole lens, and a final accelerating electrode, installed adjacent to the sixth focusing electrode and forming a main lens.
In the present invention, the first quadrupole lens is constructed such that horizontally elongated electron beam holes are formed on the exit side of the third focusing electrode, and vertically elongated electron beam holes are formed on the entrance side of the fourth focusing electrode.
The second quadrupole lens is constructed such that horizontally elongated electron beam holes are formed on the exit side of the fourth focusing electrode, vertically elongated electron beam holes are formed on the fifth focusing electrode and circular electron beam holes formed on the entrance side of the sixth focusing electrode. Also, the second quadrupole lens is constructed such that horizontally elongated electron beam holes are formed on the exit side of the fourth focusing electrode, vertically elongated electron beam holes are formed on the fifth focusing electrode and vertically elongated electron beam holes are formed on the entrance side of the sixth focusing electrode.
According to another aspect of the present invention, there is provided an electron gun for a color cathode ray tube including cathodes, a control electrode and a screen electrode, forming a triode, first and second focusing electrodes sequentially installed from the screen electrode, a third focusing electrode having horizontally elongated electron beam holes formed on its exit side, and fourth focusing electrode having vertically and horizontally elongated electron beam holes formed on its entrance and exit sides, respectively, and a fifth focusing electrode, installed adjacent to the fourth focusing electrode and having vertically elongated electron beam holes, and a final accelerating electrode installed adjacent to the fifth focusing electrode, the fifth focusing electrode and the final accelerating electrode forming a main lens, wherein a static voltage is applied to the screen electrode and the second focusing electrode, a focus voltage is applied to the first, third and fifth focusing electrodes, a parabola dynamic focus voltage, synchronized with a deflection signal, is applied to the fourth focus electrode and a sixth focusing electrode.